Method and apparatus for performing channel sensing in a wireless communication system

ABSTRACT

Provided is a method for performing channel sensing in an unlicensed band in a wireless communication system, performed by a node performing channel sensing, the method comprising: receiving information about a known signal to be transmitted by at least one node of a group to which the node performing the channel sensing belongs to; detecting the known signal using the information about the known signal; canceling reception power of the known signal from measured reception power, during a time period in which the known signal is detected; comparing remaining reception power except for the reception power of the known signal with a threshold; and determining whether a channel is occupied or unoccupied according to a result of the comparison, wherein the information about the known signal includes at least one of a sequence of the known signal, transmission power of the known signal, a valid duration of the reception power of the known signal, a power offset between a transmission signal and the known signal during the valid duration, or a cancelation ratio of reception power of the transmission signal during the valid duration.

This application claims the benefit of U.S. Provisional Application Nos.62/137,203 and 62/142,481, filed on Mar. 23, 2015 and Apr. 3, 2015,respectively, which are hereby incorporated by reference as if fully setforth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for performing channelsensing in an unlicensed band.

2. Discussion of the Related Art

Recently, various devices requiring machine-to-machine (M2M)communication and high data transfer rate, such as smartphones or tabletpersonal computers (PCs), have appeared and come into widespread use.This has rapidly increased the quantity of data which needs to beprocessed in a cellular network. In order to satisfy such rapidlyincreasing data throughput, recently, carrier aggregation (CA)technology which efficiently uses more frequency bands, cognitive ratiotechnology, multiple antenna (MIMO) technology for increasing datacapacity in a restricted frequency, multiple-base-station cooperativetechnology, etc. have been highlighted. In addition, communicationenvironments have evolved such that the density of accessible nodes isincreased in the vicinity of a user equipment (UE). Here, the nodeincludes one or more antennas and refers to a fixed point capable oftransmitting/receiving radio frequency (RF) signals to/from the userequipment (UE). A communication system including high-density nodes mayprovide a communication service of higher performance to the UE bycooperation between nodes.

A multi-node coordinated communication scheme in which a plurality ofnodes communicates with a user equipment (UE) using the sametime-frequency resources has much higher data throughput than legacycommunication scheme in which each node operates as an independent basestation (BS) to communicate with the UE without cooperation.

A multi-node system performs coordinated communication using a pluralityof nodes, each of which operates as a base station or an access point,an antenna, an antenna group, a remote radio head (RRH), and a remoteradio unit (RRU). Unlike the conventional centralized antenna system inwhich antennas are concentrated at a base station (BS), nodes are spacedapart from each other by a predetermined distance or more in themulti-node system. The nodes can be managed by one or more base stationsor base station controllers which control operations of the nodes orschedule data transmitted/received through the nodes. Each node isconnected to a base station or a base station controller which managesthe node through a cable or a dedicated line.

The multi-node system can be considered as a kind of Multiple InputMultiple Output (MIMO) system since dispersed nodes can communicate witha single UE or multiple UEs by simultaneously transmitting/receivingdifferent data streams. However, since the multi-node system transmitssignals using the dispersed nodes, a transmission area covered by eachantenna is reduced compared to antennas included in the conventionalcentralized antenna system. Accordingly, transmit power required foreach antenna to transmit a signal in the multi-node system can bereduced compared to the conventional centralized antenna system usingMIMO. In addition, a transmission distance between an antenna and a UEis reduced to decrease in pathloss and enable rapid data transmission inthe multi-node system. This can improve transmission capacity and powerefficiency of a cellular system and meet communication performancehaving relatively uniform quality regardless of UE locations in a cell.Further, the multi-node system reduces signal loss generated duringtransmission since base station(s) or base station controller(s)connected to a plurality of nodes transmit/receive data in cooperationwith each other. When nodes spaced apart by over a predetermineddistance perform coordinated communication with a UE, correlation andinterference between antennas are reduced. Therefore, a high signal tointerference-plus-noise ratio (SINR) can be obtained according to themulti-node coordinated communication scheme.

Owing to the above-mentioned advantages of the multi-node system, themulti-node system is used with or replaces the conventional centralizedantenna system to become a new foundation of cellular communication inorder to reduce base station cost and backhaul network maintenance costwhile extending service coverage and improving channel capacity and SINRin next-generation mobile communication systems.

SUMMARY OF THE INVENTION

The present invention is directed to a method for channel sensing in aunlicensed band.

According to an embodiment of the present invention, there is provided amethod for performing channel sensing in an unlicensed band in awireless communication system, performed by a node performing channelsensing, the method comprising receiving information about a knownsignal to be transmitted by at least one node of a group to which thenode performing the channel sensing belongs to; detecting the knownsignal using the information about the known signal; canceling receptionpower of the known signal from measured reception power, during a timeperiod in which the known signal is detected; comparing remainingreception power except for the reception power of the known signal witha threshold; and determining whether a channel is occupied or unoccupiedaccording to a result of the comparison, wherein the information aboutthe known signal includes at least one of a sequence of the knownsignal, transmission power of the known signal, a valid duration of thereception power of the known signal, a power offset between atransmission signal and the known signal during the valid duration, or acancelation ratio of reception power of the transmission signal duringthe valid duration.

Alternatively or additionally, the valid duration may include a subframein which the known signal is detected or a subframe in which a signal ofreception power to be derived from the reception power of the knownsignal is transmitted.

Alternatively or additionally, the known signal may include areservation signal or a preamble to be transmitted for channeloccupation after channel sensing is successful and before transmitting asignal during a channel occupancy time, or a reference signal forchannel estimation and demodulation.

Alternatively or additionally, the method may further comprise if theinformation about the known signal includes information about the validduration or the known signal indicates the valid duration, canceling thereception power of the known signal from measured reception power duringthe valid duration.

Alternatively or additionally, if the known signal is transmitted in aplurality of orthogonal frequency division multiplexing (OFDM) symbols,the known signal may be transmitted with a predetermined phasedifference in different OFDM symbols to indicate remaining lengths of atransmission burst including the known signal.

Alternatively or additionally, the method may further comprise using thereception power of the known signal in calculating reception power forthe channel sensing during a time period corresponding to the remaininglength of the transmission burst.

According to another embodiment of the present invention, there isprovided a node for performing channel sensing in an unlicensed band ina wireless communication system, the node comprising a radio frequency(RF) unit; and a processor configured to control the RF unit, whereinthe processor is configured to receive information about a known signalto be transmitted by at least one node of a group which the nodeperforming the channel sensing belongs to, detect the known signal usingthe information about the known signal, cancel reception power of theknown signal from measured reception power, during a time period inwhich the known signal is detected, compare remaining reception powerexcept for the reception power of the known signal with a threshold, anddetermine whether a channel is occupied or unoccupied according to aresult of the comparison, and

wherein the information about the known signal includes at least one ofa sequence of the known signal, transmission power of the known signal,a valid duration of the reception power of the known signal, a poweroffset between a transmission signal and the known signal during thevalid duration, or a cancelation ratio of reception power of thetransmission signal during the valid duration.

Alternatively or additionally, the valid duration may include a subframein which the known signal is detected or a subframe in which a signal ofreception power to be derived from the reception power of the knownsignal is transmitted.

Alternatively or additionally, the known signal may include areservation signal or a preamble to be transmitted for channeloccupation after channel sensing is successful and before a signal istransmitted during a channel occupancy time, or a reference signal forchannel estimation and demodulation.

Alternatively or additionally, if the information about the known signalincludes information about the valid duration or the known signal mayindicate the valid duration, the processor is configured to cancel thereception power of the known signal from measured reception power duringthe valid duration.

Alternatively or additionally, if the known signal is transmitted in aplurality of orthogonal frequency division multiplexing (OFDM) symbols,the known signal may be transmitted with a predetermined phasedifference in different OFDM symbols to indicate remaining lengths of atransmission burst including the known signal.

Alternatively or additionally, the processor may be configured to usethe reception power of the known signal in calculating reception powerfor the channel sensing during a time period corresponding to theremaining length of the transmission burst.

The aforementioned technical solutions are merely parts of embodimentsof the present invention and various embodiments in which the technicalfeatures of the present invention are reflected can be derived andunderstood by a person skilled in the art on the basis of the followingdetailed description of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 illustrates exemplary radio frame structures in a wirelesscommunication system;

FIG. 2 illustrates an exemplary structure of a downlink (DL)/uplink (UL)slot in a wireless communication system;

FIG. 3 illustrates an exemplary structure of a DL subframe in a 3^(rd)generation partnership project (3GPP) long term evolution(LTE)/LTE-advanced (LTE-A) system;

FIG. 4 illustrates an exemplary structure of a UL subframe in the 3GPPLTE/LTE-A system;

FIG. 5 illustrates an exemplary listen before talk (LBT) channel accessoperation based on frame based equipment (FBE);

FIG. 6 illustrates an exemplary an LBT channel access operation based onload based equipment (LBE);

FIG. 7 illustrates an example of canceling reception power of atransmission signal of a node within the same group according to anembodiment of the present invention;

FIG. 8 illustrates phase differences applied to a known signaltransmitted in different symbols according to an embodiment of thepresent invention;

FIG. 9 illustrates transmission power-related cooperation between nodesaccording to an embodiment of the present invention;

FIG. 10 illustrates exemplary orthogonal cover code (OCC)-based channelsensing according to an embodiment of the present invention;

FIG. 11 illustrates exemplary OCC-based channel sensing according to anembodiment of the present invention;

FIG. 12 illustrates an operation according to an embodiment of thepresent invention; and

FIG. 13 is a block diagram of devices for implementing an embodiment(s)of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The accompanying drawings illustrate exemplary embodiments ofthe present invention and provide a more detailed description of thepresent invention. However, the scope of the present invention shouldnot be limited thereto.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

In the present invention, a user equipment (UE) is fixed or mobile. TheUE is a device that transmits and receives user data and/or controlinformation by communicating with a base station (BS). The term ‘UE’ maybe replaced with ‘terminal equipment’, ‘Mobile Station (MS)’, ‘MobileTerminal (MT)’, ‘User Terminal (UT)’, ‘Subscriber Station (SS)’,‘wireless device’, ‘Personal Digital Assistant (PDA)’, ‘wireless modem’,‘handheld device’, etc. A BS is typically a fixed station thatcommunicates with a UE and/or another BS. The BS exchanges data andcontrol information with a UE and another BS. The term ‘BS’ may bereplaced with ‘Advanced Base Station (ABS)’, ‘Node B’, ‘evolved-Node B(eNB)’, ‘Base Transceiver System (BTS)’, ‘Access Point (AP)’,‘Processing Server (PS)’, etc. In the following description, BS iscommonly called eNB.

In the present invention, a node refers to a fixed point capable oftransmitting/receiving a radio signal to/from a UE by communication withthe UE. Various eNBs can be used as nodes. For example, a node can be aBS, NB, eNB, Pico-cell eNB (PeNB), home eNB (HeNB), relay, repeater,etc. Furthermore, a node may not be an eNB. For example, a node can be aradio remote head (RRH) or a radio remote unit (RRU). The RRH and RRUhave power levels lower than that of the eNB. Since the RRH or RRU(referred to as RRH/RRU hereinafter) is connected to an eNB through adedicated line such as an optical cable in general, cooperativecommunication according to RRH/RRU and eNB can be smoothly performedcompared to cooperative communication according to eNBs connectedthrough a wireless link. At least one antenna is installed per node. Anantenna may refer to an antenna port, a virtual antenna or an antennagroup. A node may also be called a point. Unlink a conventionalcentralized antenna system (CAS) (i.e. single node system) in whichantennas are concentrated in an eNB and controlled an eNB controller,plural nodes are spaced apart at a predetermined distance or longer in amulti-node system. The plural nodes can be managed by one or more eNBsor eNB controllers that control operations of the nodes or schedule datato be transmitted/received through the nodes. Each node may be connectedto an eNB or eNB controller managing the corresponding node via a cableor a dedicated line. In the multi-node system, the same cell identity(ID) or different cell IDs may be used for signal transmission/receptionthrough plural nodes. When plural nodes have the same cell ID, each ofthe plural nodes operates as an antenna group of a cell. If nodes havedifferent cell IDs in the multi-node system, the multi-node system canbe regarded as a multi-cell (e.g., macro-cell/femto-cell/pico-cell)system. When multiple cells respectively configured by plural nodes areoverlaid according to coverage, a network configured by multiple cellsis called a multi-tier network. The cell ID of the RRH/RRU may beidentical to or different from the cell ID of an eNB. When the RRH/RRUand eNB use different cell IDs, both the RRH/RRU and eNB operate asindependent eNBs.

In a multi-node system according to the present invention, which will bedescribed below, one or more eNBs or eNB controllers connected to pluralnodes can control the plural nodes such that signals are simultaneouslytransmitted to or received from a UE through some or all nodes. Whilethere is a difference between multi-node systems according to the natureof each node and implementation form of each node, multi-node systemsare discriminated from single node systems (e.g. CAS, conventional MIMOsystems, conventional relay systems, conventional repeater systems,etc.) since a plurality of nodes provides communication services to a UEin a predetermined time-frequency resource. Accordingly, embodiments ofthe present invention with respect to a method of performing coordinateddata transmission using some or all nodes can be applied to varioustypes of multi-node systems. For example, a node refers to an antennagroup spaced apart from another node by a predetermined distance ormore, in general. However, embodiments of the present invention, whichwill be described below, can even be applied to a case in which a noderefers to an arbitrary antenna group irrespective of node interval. Inthe case of an eNB including an X-pole (cross polarized) antenna, forexample, the embodiments of the preset invention are applicable on theassumption that the eNB controls a node composed of an H-pole antennaand a V-pole antenna.

A communication scheme through which signals are transmitted/receivedvia plural transmit (Tx)/receive (Rx) nodes, signals aretransmitted/received via at least one node selected from plural Tx/Rxnodes, or a node transmitting a downlink signal is discriminated from anode transmitting an uplink signal is called multi-eNB MIMO or CoMP(Coordinated Multi-Point Tx/Rx). Coordinated transmission schemes fromamong CoMP communication schemes can be categorized into JP (JointProcessing) and scheduling coordination. The former may be divided intoJT (Joint Transmission)/JR (Joint Reception) and DPS (Dynamic PointSelection) and the latter may be divided into CS (CoordinatedScheduling) and CB (Coordinated Beamforming). DPS may be called DCS(Dynamic Cell Selection). When JP is performed, more variouscommunication environments can be generated, compared to other CoMPschemes. JT refers to a communication scheme by which plural nodestransmit the same stream to a UE and JR refers to a communication schemeby which plural nodes receive the same stream from the UE. The UE/eNBcombine signals received from the plural nodes to restore the stream. Inthe case of JT/JR, signal transmission reliability can be improvedaccording to transmit diversity since the same stream is transmittedfrom/to plural nodes. DPS refers to a communication scheme by which asignal is transmitted/received through a node selected from plural nodesaccording to a specific rule. In the case of DPS, signal transmissionreliability can be improved because a node having a good channel statebetween the node and a UE is selected as a communication node.

In the present invention, a cell refers to a specific geographical areain which one or more nodes provide communication services. Accordingly,communication with a specific cell may mean communication with an eNB ora node providing communication services to the specific cell. Adownlink/uplink signal of a specific cell refers to a downlink/uplinksignal from/to an eNB or a node providing communication services to thespecific cell. A cell providing uplink/downlink communication servicesto a UE is called a serving cell. Furthermore, channel status/quality ofa specific cell refers to channel status/quality of a channel or acommunication link generated between an eNB or a node providingcommunication services to the specific cell and a UE. In 3GPP LTE-Asystems, a UE can measure downlink channel state from a specific nodeusing one or more CSI-RSs (Channel State Information Reference Signals)transmitted through antenna port(s) of the specific node on a CSI-RSresource allocated to the specific node. In general, neighboring nodestransmit CSI-RS resources on orthogonal CSI-RS resources. When CSI-RSresources are orthogonal, this means that the CSI-RS resources havedifferent subframe configurations and/or CSI-RS sequences which specifysubframes to which CSI-RSs are allocated according to CSI-RS resourceconfigurations, subframe offsets and transmission periods, etc. whichspecify symbols and subcarriers carrying the CSI RSs.

In the present invention, PDCCH (Physical Downlink ControlChannel)/PCFICH (Physical Control Format Indicator Channel)/PHICH(Physical Hybrid automatic repeat request Indicator Channel)/PDSCH(Physical Downlink Shared Channel) refer to a set of time-frequencyresources or resource elements respectively carrying DCI (DownlinkControl Information)/CFI (Control Format Indicator)/downlink ACK/NACK(Acknowledgement/Negative ACK)/downlink data. In addition, PUCCH(Physical Uplink Control Channel)/PUSCH (Physical Uplink SharedChannel)/PRACH (Physical Random Access Channel) refer to sets oftime-frequency resources or resource elements respectively carrying UCI(Uplink Control Information)/uplink data/random access signals. In thepresent invention, a time-frequency resource or a resource element (RE),which is allocated to or belongs toPDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH, is referred to as aPDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH RE orPDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH resource. In the followingdescription, transmission of PUCCH/PUSCH/PRACH by a UE is equivalent totransmission of uplink control information/uplink data/random accesssignal through or on PUCCH/PUSCH/PRACH. Furthermore, transmission ofPDCCH/PCFICH/PHICH/PDSCH by an eNB is equivalent to transmission ofdownlink data/control information through or onPDCCH/PCFICH/PHICH/PDSCH.

FIG. 1 illustrates an exemplary radio frame structure used in a wirelesscommunication system. FIG. 1(a) illustrates a frame structure forfrequency division duplex (FDD) used in 3GPP LTE/LTE-A and FIG. 1(b)illustrates a frame structure for time division duplex (TDD) used in3GPP LTE/LTE-A.

Referring to FIG. 1, a radio frame used in 3GPP LTE/LTE-A has a lengthof 10 ms (307200 Ts) and includes 10 subframes in equal size. The 10subframes in the radio frame may be numbered. Here, Ts denotes samplingtime and is represented as Ts=1/(2048*15 kHz). Each subframe has alength of 1 ms and includes two slots. 20 slots in the radio frame canbe sequentially numbered from 0 to 19. Each slot has a length of 0.5 ms.A time for transmitting a subframe is defined as a transmission timeinterval (TTI). Time resources can be discriminated by a radio framenumber (or radio frame index), subframe number (or subframe index) and aslot number (or slot index).

The radio frame can be configured differently according to duplex mode.Downlink transmission is discriminated from uplink transmission byfrequency in FDD mode, and thus the radio frame includes only one of adownlink subframe and an uplink subframe in a specific frequency band.In TDD mode, downlink transmission is discriminated from uplinktransmission by time, and thus the radio frame includes both a downlinksubframe and an uplink subframe in a specific frequency band.

Table 1 shows DL-UL configurations of subframes in a radio frame in theTDD mode.

TABLE 1 Downlink- DL-UL to-Uplink configu- Switch-point Subframe numberration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 msD S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms  D S U U U D D DD D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D D D D 6 5 ms D SU U U D S U U D

In Table 1, D denotes a downlink subframe, U denotes an uplink subframeand S denotes a special subframe. The special subframe includes threefields of DwPTS (Downlink Pilot TimeSlot), GP (Guard Period), and UpPTS(Uplink Pilot TimeSlot). DwPTS is a period reserved for downlinktransmission and UpPTS is a period reserved for uplink transmission.Table 2 shows special subframe configuration.

TABLE 2 Normal cyclic prefix in downlink Extended cyclic prefix indownlink Special UpPTS UpPTS subframe Normal cyclic Extended cyclicNormal cyclic Extended cyclic configuration DwPTS prefix in uplinkprefix in uplink DwPTS prefix in uplink prefix in uplink 0  6592 · T_(s)2192 · T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 119760 · T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 ·T_(s) 25600 · T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 ·T_(s) 5  6592 · T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 ·T_(s) 23040 · T_(s) 7 21952 · T_(s) 12800 · T_(s) 8 24144 · T_(s) — — —9 13168 · T_(s) — — —

FIG. 2 illustrates an exemplary downlink/uplink slot structure in awireless communication system. Particularly, FIG. 2 illustrates aresource grid structure in 3GPP LTE/LTE-A. A resource grid is presentper antenna port.

Referring to FIG. 2, a slot includes a plurality of OFDM (OrthogonalFrequency Division Multiplexing) symbols in the time domain and aplurality of resource blocks (RBs) in the frequency domain. An OFDMsymbol may refer to a symbol period. A signal transmitted in each slotmay be represented by a resource grid composed of N_(RB) ^(DL/UL)*N_(sc)^(RB) subcarriers and N_(symb) ^(DL/UL) OFDM symbols. Here, N_(RB) ^(DL)denotes the number of RBs in a downlink slot and N_(RB) ^(UL) denotesthe number of RBs in an uplink slot. N_(RB) ^(DL) and N_(RB) ^(UL)respectively depend on a DL transmission bandwidth and a UL transmissionbandwidth. N_(symb) ^(DL) denotes the number of OFDM symbols in thedownlink slot and N_(symb) ^(UL) denotes the number of OFDM symbols inthe uplink slot. In addition, N_(sc) ^(RB) denotes the number ofsubcarriers constructing one RB.

An OFDM symbol may be called an SC-FDM (Single Carrier FrequencyDivision Multiplexing) symbol according to multiple access scheme. Thenumber of OFDM symbols included in a slot may depend on a channelbandwidth and the length of a cyclic prefix (CP). For example, a slotincludes 7 OFDM symbols in the case of normal CP and 6 OFDM symbols inthe case of extended CP. While FIG. 2 illustrates a subframe in which aslot includes 7 OFDM symbols for convenience, embodiments of the presentinvention can be equally applied to subframes having different numbersof OFDM symbols. Referring to FIG. 2, each OFDM symbol includes N_(RB)^(DL/UL)*N_(sc) ^(RB) subcarriers in the frequency domain. Subcarriertypes can be classified into a data subcarrier for data transmission, areference signal subcarrier for reference signal transmission, and nullsubcarriers for a guard band and a direct current (DC) component. Thenull subcarrier for a DC component is a subcarrier remaining unused andis mapped to a carrier frequency (f0) during OFDM signal generation orfrequency up-conversion. The carrier frequency is also called a centerfrequency.

An RB is defined by N_(symb) ^(DL/UL) (e.g., 7) consecutive OFDM symbolsin the time domain and N_(sc) ^(RB) (e.g., 12) consecutive subcarriersin the frequency domain. For reference, a resource composed by an OFDMsymbol and a subcarrier is called a resource element (RE) or a tone.Accordingly, an RB is composed of N_(symb) ^(DL/UL)*N_(sc) ^(RB) REs.Each RE in a resource grid can be uniquely defined by an index pair(k, 1) in a slot. Here, k is an index in the range of 0 to N_(symb)^(DL/UL)*N_(sc) ^(RB)−1 in the frequency domain and 1 is an index in therange of 0 to N_(symb) ^(DL/UL)−1.

Two RBs that occupy N_(sc) ^(RB) consecutive subcarriers in a subframeand respectively disposed in two slots of the subframe are called aphysical resource block (PRB) pair. Two RBs constituting a PRB pair havethe same PRB number (or PRB index). A virtual resource block (VRB) is alogical resource allocation unit for resource allocation. The VRB hasthe same size as that of the PRB. The VRB may be divided into alocalized VRB and a distributed VRB depending on a mapping scheme of VRBinto PRB. The localized VRBs are mapped into the PRBs, whereby VRBnumber (VRB index) corresponds to PRB number. That is, nPRB=nVRB isobtained. Numbers are given to the localized VRBs from 0 to N_(VRB)^(DL)−1, and N_(VRB) ^(DL)=N_(RB) ^(DL) is obtained. Accordingly,according to the localized mapping scheme, the VRBs having the same VRBnumber are mapped into the PRBs having the same PRB number at the firstslot and the second slot. On the other hand, the distributed VRBs aremapped into the PRBs through interleaving. Accordingly, the VRBs havingthe same VRB number may be mapped into the PRBs having different PRBnumbers at the first slot and the second slot. Two PRBs, which arerespectively located at two slots of the subframe and have the same VRBnumber, will be referred to as a pair of VRBs.

FIG. 3 illustrates a downlink (DL) subframe structure used in 3GPPLTE/LTE-A.

Referring to FIG. 3, a DL subframe is divided into a control region anda data region. A maximum of three (four) OFDM symbols located in a frontportion of a first slot within a subframe correspond to the controlregion to which a control channel is allocated. A resource regionavailable for PDCCH transmission in the DL subframe is referred to as aPDCCH region hereinafter. The remaining OFDM symbols correspond to thedata region to which a physical downlink shared chancel (PDSCH) isallocated. A resource region available for PDSCH transmission in the DLsubframe is referred to as a PDSCH region hereinafter. Examples ofdownlink control channels used in 3GPP LTE include a physical controlformat indicator channel (PCFICH), a physical downlink control channel(PDCCH), a physical hybrid ARQ indicator channel (PHICH), etc. ThePCFICH is transmitted at a first OFDM symbol of a subframe and carriesinformation regarding the number of OFDM symbols used for transmissionof control channels within the subframe. The PHICH is a response ofuplink transmission and carries an HARQ acknowledgment (ACK)/negativeacknowledgment (NACK) signal.

Control information carried on the PDCCH is called downlink controlinformation (DCI). The DCI contains resource allocation information andcontrol information for a UE or a UE group. For example, the DCIincludes a transport format and resource allocation information of adownlink shared channel (DL-SCH), a transport format and resourceallocation information of an uplink shared channel (UL-SCH), paginginformation of a paging channel (PCH), system information on the DL-SCH,information about resource allocation of an upper layer control messagesuch as a random access response transmitted on the PDSCH, a transmitcontrol command set with respect to individual UEs in a UE group, atransmit power control command, information on activation of a voiceover IP (VoIP), downlink assignment index (DAI), etc. The transportformat and resource allocation information of the DL-SCH are also calledDL scheduling information or a DL grant and the transport format andresource allocation information of the UL-SCH are also called ULscheduling information or a UL grant. The size and purpose of DCIcarried on a PDCCH depend on DCI format and the size thereof may bevaried according to coding rate. Various formats, for example, formats 0and 4 for uplink and formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3 and 3Afor downlink, have been defined in 3GPP LTE. Control information such asa hopping flag, information on RB allocation, modulation coding scheme(MCS), redundancy version (RV), new data indicator (NDI), information ontransmit power control (TPC), cyclic shift demodulation reference signal(DMRS), UL index, channel quality information (CQI) request, DLassignment index, HARQ process number, transmitted precoding matrixindicator (TPMI), precoding matrix indicator (PMI), etc. is selected andcombined based on DCI format and transmitted to a UE as DCI.

In general, a DCI format for a UE depends on transmission mode (TM) setfor the UE. In other words, only a DCI format corresponding to aspecific TM can be used for a UE configured in the specific TM.

A PDCCH is transmitted on an aggregation of one or several consecutivecontrol channel elements (CCEs). The CCE is a logical allocation unitused to provide the PDCCH with a coding rate based on a state of a radiochannel. The CCE corresponds to a plurality of resource element groups(REGs). For example, a CCE corresponds to 9 REGs and an REG correspondsto 4 REs. 3GPP LTE defines a CCE set in which a PDCCH can be located foreach UE. A CCE set from which a UE can detect a PDCCH thereof is calleda PDCCH search space, simply, search space. An individual resourcethrough which the PDCCH can be transmitted within the search space iscalled a PDCCH candidate. A set of PDCCH candidates to be monitored bythe UE is defined as the search space. In 3GPP LTE/LTE-A, search spacesfor DCI formats may have different sizes and include a dedicated searchspace and a common search space. The dedicated search space is aUE-specific search space and is configured for each UE. The commonsearch space is configured for a plurality of UEs. Aggregation levelsdefining the search space is as follows.

TABLE 3 Search Space Aggregation Level Number of PDCCH Type L Size [inCCEs] candidates M^((L)) UE-specific 1 6 6 2 12 6 4 8 2 8 16 2 Common 416 4 8 16 2

A PDCCH candidate corresponds to 1, 2, 4 or 8 CCEs according to CCEaggregation level. An eNB transmits a PDCCH (DCI) on an arbitrary PDCCHcandidate with in a search space and a UE monitors the search space todetect the PDCCH (DCI). Here, monitoring refers to attempting to decodeeach PDCCH in the corresponding search space according to all monitoredDCI formats. The UE can detect the PDCCH thereof by monitoring pluralPDCCHs. Since the UE does not know the position in which the PDCCHthereof is transmitted, the UE attempts to decode all PDCCHs of thecorresponding DCI format for each subframe until a PDCCH having the IDthereof is detected. This process is called blind detection (or blinddecoding (BD)).

The eNB can transmit data for a UE or a UE group through the dataregion. Data transmitted through the data region may be called userdata. For transmission of the user data, a physical downlink sharedchannel (PDSCH) may be allocated to the data region. A paging channel(PCH) and downlink-shared channel (DL-SCH) are transmitted through thePDSCH. The UE can read data transmitted through the PDSCH by decodingcontrol information transmitted through a PDCCH. Informationrepresenting a UE or a UE group to which data on the PDSCH istransmitted, how the UE or UE group receives and decodes the PDSCH data,etc. is included in the PDCCH and transmitted. For example, if aspecific PDCCH is CRC (cyclic redundancy check)-masked having radionetwork temporary identify (RNTI) of “A” and information about datatransmitted using a radio resource (e.g., frequency position) of “B” andtransmission format information (e.g., transport block size, modulationscheme, coding information, etc.) of “C” is transmitted through aspecific DL subframe, the UE monitors PDCCHs using RNTI information anda UE having the RNTI of “A” detects a PDCCH and receives a PDSCHindicated by “B” and “C” using information about the PDCCH.

A reference signal (RS) to be compared with a data signal is necessaryfor the UE to demodulate a signal received from the eNB. A referencesignal refers to a predetermined signal having a specific waveform,which is transmitted from the eNB to the UE or from the UE to the eNBand known to both the eNB and UE. The reference signal is also called apilot. Reference signals are categorized into a cell-specific RS sharedby all UEs in a cell and a modulation RS (DM RS) dedicated for aspecific UE. A DM RS transmitted by the eNB for demodulation of downlinkdata for a specific UE is called a UE-specific RS. Both or one of DM RSand CRS may be transmitted on downlink. When only the DM RS istransmitted without CRS, an RS for channel measurement needs to beadditionally provided because the DM RS transmitted using the sameprecoder as used for data can be used for demodulation only. Forexample, in 3GPP LTE(-A), CSI-RS corresponding to an additional RS formeasurement is transmitted to the UE such that the UE can measurechannel state information. CSI-RS is transmitted in each transmissionperiod corresponding to a plurality of subframes based on the fact thatchannel state variation with time is not large, unlike CRS transmittedper subframe.

FIG. 4 illustrates an exemplary uplink subframe structure used in 3GPPLTE/LTE-A.

Referring to FIG. 4, a UL subframe can be divided into a control regionand a data region in the frequency domain. One or more PUCCHs (physicaluplink control channels) can be allocated to the control region to carryuplink control information (UCI). One or more PUSCHs (Physical uplinkshared channels) may be allocated to the data region of the UL subframeto carry user data.

In the UL subframe, subcarriers spaced apart from a DC subcarrier areused as the control region. In other words, subcarriers corresponding toboth ends of a UL transmission bandwidth are assigned to UCItransmission. The DC subcarrier is a component remaining unused forsignal transmission and is mapped to the carrier frequency f0 duringfrequency up-conversion. A PUCCH for a UE is allocated to an RB pairbelonging to resources operating at a carrier frequency and RBsbelonging to the RB pair occupy different subcarriers in two slots.Assignment of the PUCCH in this manner is represented as frequencyhopping of an RB pair allocated to the PUCCH at a slot boundary. Whenfrequency hopping is not applied, the RB pair occupies the samesubcarrier.

The PUCCH can be used to transmit the following control information.

-   -   Scheduling Request (SR): This is information used to request a        UL-SCH resource and is transmitted using On-Off Keying (OOK)        scheme.    -   HARQ ACK/NACK: This is a response signal to a downlink data        packet on a PDSCH and indicates whether the downlink data packet        has been successfully received. A 1-bit ACK/NACK signal is        transmitted as a response to a single downlink codeword and a        2-bit ACK/NACK signal is transmitted as a response to two        downlink codewords. HARQ-ACK responses include positive ACK        (ACK), negative ACK (HACK), discontinuous transmission (DTX) and        NACK/DTX. Here, the term HARQ-ACK is used interchangeably with        the term HARQ ACK/NACK and ACK/NACK.    -   Channel State Indicator (CSI): This is feedback information        about a downlink channel. Feedback information regarding MIMO        includes a rank indicator (RI) and a precoding matrix indicator        (PMI).

The quantity of control information (UCI) that a UE can transmit througha subframe depends on the number of SC-FDMA symbols available forcontrol information transmission. The SC-FDMA symbols available forcontrol information transmission correspond to SC-FDMA symbols otherthan SC-FDMA symbols of the subframe, which are used for referencesignal transmission. In the case of a subframe in which a soundingreference signal (SRS) is configured, the last SC-FDMA symbol of thesubframe is excluded from the SC-FDMA symbols available for controlinformation transmission. A reference signal is used to detect coherenceof the PUCCH. The PUCCH supports various formats according toinformation transmitted thereon.

Table 4 shows the mapping relationship between PUCCH formats and UCI inLTE/LTE-A.

TABLE 4 Number of bits per PUCCH Modulation subframe, format schemeM_(bit) Usage Etc. 1 N/A N/A SR (Scheduling Request) 1a BPSK 1 ACK/NACKor One SR + ACK/NACK codeword 1b QPSK 2 ACK/NACK or Two SR + ACK/NACKcodeword 2 QPSK 20 CQI/PMI/RI Joint coding ACK/NACK (extended CP) 2aQPSK + BPSK 21 CQI/PMI/RI + Normal CP ACK/NACK only 2b QPSK + QPSK 22CQI/PMI/RI + Normal CP ACK/NACK only 3 QPSK 48 ACK/NACK or SR + ACK/NACKor CQI/PMI/RI + ACK/NACK

Referring to Table 4, PUCCH formats 1/1a/1b are used to transmitACK/NACK information, PUCCH format 2/2a/2b are used to carry CSI such asCQI/PMI/RI and PUCCH format 3 is used to transmit ACK/NACK information.

Reference Signal (RS)

When a packet is transmitted in a wireless communication system, signaldistortion may occur during transmission since the packet is transmittedthrough a radio channel. To correctly receive a distorted signal at areceiver, the distorted signal needs to be corrected using channelinformation. To detect channel information, a signal known to both atransmitter and the receiver is transmitted and channel information isdetected with a degree of distortion of the signal when the signal isreceived through a channel. This signal is called a pilot signal or areference signal.

When data is transmitted/received using multiple antennas, the receivercan receive a correct signal only when the receiver is aware of achannel state between each transmit antenna and each receive antenna.Accordingly, a reference signal needs to be provided per transmitantenna, more specifically, per antenna port.

Reference signals can be classified into an uplink reference signal anda downlink reference signal. In LTE, the uplink reference signalincludes:

i) a demodulation reference signal (DMRS) for channel estimation forcoherent demodulation of information transmitted through a PUSCH and aPUCCH; and

ii) a sounding reference signal (SRS) used for an eNB to measure uplinkchannel quality at a frequency of a different network.

The downlink reference signal includes:

i) a cell-specific reference signal (CRS) shared by all UEs in a cell;

ii) a UE-specific reference signal for a specific UE only;

iii) a DMRS transmitted for coherent demodulation when a PDSCH istransmitted;

iv) a channel state information reference signal (CSI-RS) for deliveringchannel state information (CSI) when a downlink DMRS is transmitted;

v) a multimedia broadcast single frequency network (MBSFN) referencesignal transmitted for coherent demodulation of a signal transmitted inMB SFN mode; and

vi) a positioning reference signal used to estimate geographic positioninformation of a UE.

Reference signals can be classified into a reference signal for channelinformation acquisition and a reference signal for data demodulation.The former needs to be transmitted in a wide band as it is used for a UEto acquire channel information on downlink transmission and received bya UE even if the UE does not receive downlink data in a specificsubframe. This reference signal is used even in a handover situation.The latter is transmitted along with a corresponding resource by an eNBwhen the eNB transmits a downlink signal and is used for a UE todemodulate data through channel measurement. This reference signal needsto be transmitted in a region in which data is transmitted.

Along with a rapid increase in data traffic attributed to the recentproliferation of smartphones, a future-generation wireless communicationsystem such as 3GPP LTE-A seeks to efficiently utilize a limitedfrequency band. In this context, operating a cellular network such as anLTE system in an unlicensed band of 2.4 GHz or 5 GHz is underconsideration.

Since it is basically assumed that wireless transmission and receptionis performed in an unlicensed band through contention betweencommunication nodes, each communication node is requested to make surethat another communication node is not transmitting a signal in theunlicensed band, by channel sensing before transmitting a signal. Forthe convenience of description, this operation is called listen beforetalk (LBT) and particularly, an operation for determining whether anyother communication node is transmitting a signal is defined as carriersensing (CS) or clear channel assessment (CCA). If it is determined as aresult of CCA that there is no other communication node transmitting asignal, this state is defined as a channel unoccupied state. On thecontrary, if there is any other communication node transmitting asignal, this state is defined as a channel occupied state. If an eNB ora UE is to transmit a signal in an unlicensed band in an LTE system, theeNB or the UE should perform LBT. While the eNB or the UE istransmitting a signal, nodes conforming to other communication standardssuch as Wi-Fi should not interfere with the eNB or the UE by performingLBT. For example, a Wi-Fi standard (e.g., 802.11ac) regulates that a CCAthreshold is −62 dBm for a non-Wi-Fi signal and −82 dBm for a Wi-Fisignal. This means that upon receipt of a non-Wi-Fi signal with power(or energy) equal to or higher than −62 dBm, a station (STA) or anaccess point (AP) does not transmit a signal in order not to causeinterference.

For example, two LBT-based channel access mechanisms are defined inEurope, frame based equipment (FBE) and load based equipment (LBE). InFBE, one frame is comprised of a channel occupancy time (e.g., 1 to 10ms) being a time period over which a communication node succeeding inchannel access may continue transmission, and an idle time being atleast 5% of the channel occupancy time, and CCA is defined as anoperation for monitoring a channel during at least 20 μs at the lastpart of the idle time. A communication node periodically performs CCA ona frame basis. If the channel is unoccupied, the communication nodetransmits data during the channel occupancy time. On the other hand, ifthe channel is occupied, the communication node defers transmission andwaits until a CCA slot of the next period. FIG. 5 illustrates an exampleof the FBE operation.

In LBE, the communication node first sets q (qε{4, 5, . . . , 32}) andthen performs CCA for one slot. If the channel is unoccupied in a firstCCA slot, the communication node may transmit data by securing a channeloccupancy time of (13/32)q ms. On the contrary, if the channel isoccupied in the first CCA slot, the communication node selects N (Nε{1,2, . . . , q}) arbitrarily (i.e., randomly) and stores the selected Nvalue as an initial value for a counter. Then, the communication nodesenses a channel state on a CCA slot basis. Each time the channel isunoccupied in one specific CCA slot, the communication node decreasesthe value of the counter by 1. If the value of the counter is 0, thecommunication node (or UE) may transmit data during the channeloccupancy time of (13/32)q ms. FIG. 6 illustrates an example of the LBEoperation.

In the above examples, the channel occupied state or the channelunoccupied state may be determined by checking whether reception powerexceeds a predetermined threshold in a CCA slot.

In general, a wireless business operator divides a spatial area intocells each having appropriate coverage and enables an eNB of each cellto conduct wireless communication with UEs within the cell, therebyminimizing interference between cells and thus allowing simultaneoustransmissions between neighbor cells. The resulting maximization offrequency reuse (FR) may increase overall system performance. The FRmaximization operation may also be preferred when the LTE systemoperates in an unlicensed band. However, since LTE nodes (e.g., eNBs orUEs) transmit signals in the unlicensed band by LBT, if a specific nodeis first occupying a channel and transmitting a signal on the channel,the other nodes are highly likely not to reuse the frequency,determining that the channel is occupied according to their CCA results.

From the perspective of maximizing FR in the unlicensed band in theLBT-based LTE system, it is preferable that a node group (e.g., eNBsmanaged by the same business operator) capable of (or recommended for)simultaneous transmissions is defined, each node of the node groupshould be able to recognize signal transmission from another node withinthe group and cancel power (or energy) of the signal transmission duringpower detection (or energy detection) for CCA. In one method, a node ofthe node group recommended for simultaneous transmissions may transmit aknown signal during signal transmission, and other nodes of the groupmay recognize the known signal, eliminate the known signal, and thenperform power detection (or energy detection) in a CCA slot. That is,the other nodes may cancel power (or energy) of the known signaltransmitted by the node of the group during CCA.

However, the above operation has its limitations in that only when CCAis performed in the presence of the known signal, the power (or energy)of the known signal may be canceled during the CCA. Accordingly, thepresent invention proposes that if there are nodes in a node groupcapable of (or recommended for) simultaneous transmissions and at leastone of the nodes is able to transmit a known signal, information aboutthe known signal (for example, power, a sequence, a power offset, etc.of the known signal) is shared among the nodes, and reception power (orenergy) for the node transmitting the known signal is canceled using theinformation about the known signal, at a non-transmission time of theknown signal during CCA. Additionally, a method for using an orthogonalcover code (OCC) to reduce the complexity of detection of a known signalis proposed.

CCA Scheme Based on Power Information about Known Signal, for FRMaximization

According to a specific embodiment of the present invention, a nodetransmitting a known signal (i.e., a transmitting node) in a node groupcapable of (or recommended for) simultaneous transmissions may indicateat least one of the following pieces of information to the other nodesof the node group:

-   -   a sequence of the known signal (for example, a seed value for        generation of the sequence);    -   (transmission) power of the known signal;    -   a valid time (or a vaild duration) of transmitting a signal of        which the reception power may be derived from reception power of        the known signal (the valid time may be defined based on a        detection time of the known signal);    -   a power offset between the transmission signal and the known        signal at the valid time; and    -   a cancellation ratio (e.g., 0≦α≦1) of reception power of the        transmission signal at the valid time.

It is proposed that upon detection of the known signal transmitted bythe specific node of the group, the other nodes of the node group, whichperform CCA, recognize the valid time based on the detection time of theknown signal, calculate reception power (or energy) of the transmissionsignal of the transmitting node at the valid time using ‘the poweroffset between the transmission signal and the known signal at the validtime’, and cancel all of the calculated reception power or as much ofthe calculated reception power as ‘the cancelation ratio of thereception power of the transmission signal at the valid time’ during theCCA.

If the CCA-performing nodes detect a plurality of known signals, thenodes may determine reception power from nodes transmitting the knownsignals using power offsets between transmission signals and the knownsignals at valid times and sequentially cancel the reception power ofthe transmitting nodes in total reception power.

If the remaining reception power (or energy) after the cancellation islower than a predetermined CCA threshold, the CCA-performing nodes mayperform signal transmission during a channel occupancy time, determiningthe channel is unoccupied.

The valid time may include a transmission time of the known signal, andthe known signal may be a reservation signal (or preamble) that the nodetransmits to occupy the channel from a time of CCA success to beforedata transmission, or an RS (e.g., CRS, DM-RS, or CSI-RS) used forchannel estimation and data demodulation.

For example, it is assumed that eNB₁ and eNB₂ belong to a node grouprecommended for simultaneous transmissions and share information aboutan aforementioned reservation signal, that is, a known signal. It isalso assumed that eNB₁ transmits a transmission (Tx) burst with the sametransmission power as the reservation signal, and the Tx burst is orcorresponds to the ‘valid time’. Then, a Wi-Fi AP and eNB₁ transmit Txbursts each having length T₀ at time t and time t+T₀/2, respectively.When eNB₁ transmits a reservation signal at a former part of the Txburst, that is, during a time period from t+T₀/2 to t+T₀/2+4, eNB₂ maycancel measured reception power P₀ of the reservation signal from eNB₁in total reception power received during a time period from t+T₀/2+4 tot+3T₀/2 and then perform CCA. As a consequence, eNB₂ may start signaltransmission after time t+T₀, determining that the channel isunoccupied.

According to an embodiment of the present invention, the valid time maybe a subframe (SF) in which the known signal is detected or an SF inwhich the reception power of the known signal is determined to be validin the LTE system. For example, if the known signal is a CRS, the validtime may be an SF in which the CRS is detected or its adjacent SF. Inthis case, the present invention may be described as a method fordetermining reception power from a specific node in other symbols of anSF in which a known signal of the specific node is detected (or otherSFs in which the known signal is expected to be valid) using thereception power of the known signal and canceling the determinedreception power during CCA.

As specific examples of an operation for canceling measured power of aknown signal during CCA in the present invention, the followingoperations may be considered.

-   -   In general, cancellation of measured power of a known signal in        measured total reception power during CCA may mean an operation        for multiplying the measured total reception power by a specific        weight (e.g., a) and subtracting the product from the total        reception power.    -   The power of the known signal may be canceled in reception power        on the assumption that power equal to or higher than the power        of the known signal is transmitted during a specific time period        following the known signal.    -   The power of the known signal may be canceled in measured        reception power at the boundary of a subframe starting after        measurement of the known signal, in a first subframe from an        available data transmission starting time, or in a first minimum        transmission unit length during CCA in a system where subframes        are synchronous, whereas the power of the known signal may be        canceled in the measured reception power in one subframe length        or a minimum transmission length unit after detection of the        known signal during CCA in a system where subframes are        asynchronous.

Method for Indicating Tx Burst Interval by Applying DifferentialEncoding to Known Signal

According to a specific embodiment of the present invention, it isproposed that if a transmitting node transmits a Tx burst including oneor more transmission units (e.g., SFs) by LBT and transmits a knownsignal in two or more symbols in each transmission unit, thetransmitting node indicates information about the remaining transmissionunits of the Tx burst after a current transmission unit bydifferentially encoding phase differences between a plurality of knownsignals in each transmission unit. That is, if 2×M known signals existin each transmission unit and B-bit information is carried by one phasedifference, the length of the remaining transmission units may beindicated in B×M bits using M phase differences. An arbitrary node maydetermine the length of the remaining transmission units of the Tx bursttransmitted by the node transmitting the known signals by determiningthe transmission units carrying the known signals by a correlation valueof a received signal using information about the sequence of the knownsignals of the transmitting node and detecting M phase difference valuesapplied to 2×M known signals in the transmission unit based oninformation about a preset (or agreed) candidate group of phasedifferences. The known signals may be reservation signals (or preambles)that the node transmits to occupy a channel during a time period from atime of CCA success to before data transmission, or RSs (e.g., CRSs,DM-RSs, or CSI-RSs) used for channel estimation and data demodulation.

For example, if the known signals are CRSs, a CRS for antenna port 0 maybe transmitted in one SF in the LTE system, as illustrated in FIG. 8.

A phase difference expressed as exp^(iθ1) is applied between a CRStransmitted in a first OFDM symbol and a CRS transmitted in a 5^(th)OFDM symbol, and a phase difference expressed as exp^(iθ2) is appliedbetween a CRS transmitted in an 8^(th) OFDM symbol and a CRS transmittedin an 11^(th) OFDM symbol, thereby indicating the remaining length ofthe Tx burst on an SF basis.

TABLE 5 Remained Tx Burst θ1 θ1 [SF] 0 0 0 0 π 1 π 0 2 π π 3

If the remaining length of the Tx burst is 0 SF, this means that thecurrent SF is the last SF of the Tx burst.

The above differential encoding-based operation is performed to, when anode performing CCA cancels reception power of a transmission signal ofa transmitting node in a Tx burst transmitted by the transmitting node,using reception power (or energy) of a known signal transmitted by thetransmitting node, determine the length of the Tx burst and supportcancelation of the reception power in the Tx burst.

Method for Adjusting Power of Known Signal, for FR Maximization

According to a specific embodiment, a node transmitting a known signalin a node group capable of (or recommended for) simultaneoustransmissions may indicate at least one of the following pieces ofinformation to the other nodes of the node group:

-   -   a sequence of the known signal (e.g., a seed value for        generation of the sequence);    -   transmission power of the known signal;    -   a transmission time or transmission period of the known signal;        and    -   power offset information that may be changed based on current        transmission power of the known signal.

Each node of the node group may detect a known signal transmitted byanother node of the node group, measure reception power (or energy) ofthe known signal, and reduce transmission power in consideration ofeffects on a CCA threshold of the transmitting node that the node willexert, or indicate at least one of the following pieces of informationto the transmitting node:

-   -   a transmission power change request (e.g., power offset): the        transmission power change request may be transmitted only when        total reception power is changed to or below a CCA threshold;    -   a CCA threshold of the node and reception power of the detected        known signal, or a difference between the CCA threshold of the        node and the reception power (or energy) of the detected known        signal; and    -   a contribution ratio of the reception power of the known signal        to total reception power. The total reception power may be        calculated for a time period during which the node transmitting        the known signal transmits a signal.

If each node of the node group receives a transmission power changerequest from another node of the node group, the node may apply a poweroffset value to transmission power of its known signal (or transmissionpower of an arbitrary signal).

The known signal may be a reservation signal (or preamble) which thenode transmits for channel occupation during a time period spanning froma time of CCA success until to before data transmission, or an RS (e.g.,CRS, DM-RS, CSI-RS, or discovery reference signal (DRS)) used forchannel estimation and data demodulation.

For example, it is assumed that eNB₁ and eNB₂ belong to a node grouprecommended for simultaneous transmissions, and when eNB₁ transmits asignal to eNB₂ because eNB₁ and eNB₂ are sufficiently near to eachother, eNB₂ does not perform signal transmission, determining thatreception power from eNB₁ exceeds a CCA threshold. According to anembodiment of the present invention, eNB₁ may estimate pathloss byreceiving a known signal from eNB₂ and thus determine the magnitude ofinterference that a signal transmitted by eNB₁ causes to eNB₂. Then,eNB₁ may help eNB₂ to perform signal transmission by reducing itstransmission power (i.e., the reception power from eNB₁ measured by eNB₂may become lower than a CCA threshold). FIG. 9 illustrates an example ofthis operation.

Or eNB₁ may indicate a range in which eNB₁ may change its transmissionpower to eNB₂, and eNB₂ may request reduction of transmission power toeNB₁, for CCA success of eNB₂ in FIG. 9.

Method for Performing CCA Based on Orthogonal Cover Code (OCC)

According to a specific embodiment of the present invention, a networkmay configure an OCC set with OCCs of length B for a node group capableof (or recommended for) simultaneous transmissions, and may configure anOCC set (e.g., a first OCC group or OCC group 1) for use in transmissionof a known signal from the node group and an OCC set (e.g., a second OCCgroup or OCC group 2) for use in CCA of the node group. Herein, the OCCsof OCC group 1 are different from the OCCs of OCC group 2, and any OCCof OCC group 1 is orthogonal to any OCC of OCC group 2.

A node transmitting a known signal may transmit the known signal (e.g.,x) in N consecutive time slots by applying one OCC of OCC group 1 (e.g.,c[n], n=0, 1, . . . , N−1) to the known signal.

Each of nodes performing CCA may perform at least one of the followingoperations.

-   -   The node sums signals by applying one OCC of OCC group 2 to the        N time slots and then performs power detection (or energy        detection) for CCA.    -   If power (or energy) detected by the above OCC-based power        detection is lower than a preset CCA threshold,        -   the node performs signal transmission during a channel            occupancy time,        -   The node determines whether there is a known signal of its            node group in the N time slots.            -   The node attempts to detect a known signal of its node                group on a time slot basis.                -   If the known signal is detected, the node calculates                    reception power (or energy) from which reception                    power (or energy) of the known signal is canceled in                    a corresponding time slot.                -   if the known signal is not detected, the node                    calculates reception power (or energy) for the time                    slot.            -   Subsequently,                -   if the sum of reception power (or energy) of each                    time slot for the N time slots is lower than a                    preset CCA threshold, the node transmits a signal                    after the last time slot.                -   if reception power (or energy) of the last time slot                    is lower than the preset CCA threshold, the node                    transmits a signal after the last time slot.    -   If the power (or energy) detected by the OCC-based power        detection is higher than the preset CCA threshold,        -   the node defers signal transmission, determining that the            channel is occupied.

The known signal may be a reservation signal (or preamble) that the nodetransmits for channel occupation during a time period spanning from atime of CCA success to before data transmission, or an RS (e.g., CRS,DM-RS, or CSI-RS) for channel estimation and data demodulation. The Ntime slots may be N CCA sub-slots divided from one CCA slot.

As in the above-described OCC-based CCA method, if OCCs for transmissionof known signals are configured to be orthogonal to OCCs for performingCCA, a known signal transmitted by a specific node of a node groupcapable of (or recommended for) simultaneous transmissions with an STAis canceled by an OCC and thus the STA does not reflect the known signalin reception power (or energy) during CCA. However, in the case wherethe OCC-based CCA method is implemented in the LTE system, without anadditional adjustment, a known signal may be canceled by a signaltransmitted by a network managed by a different business operator.Therefore, if the power detected by the OCC-applied power detection islower than the preset CCA threshold, additional power detection may beperformed, as described above.

In this manner, a specific node may efficiently exclude a signaltransmitted by a node of a node group capable of simultaneoustransmission with the specific node from CCA.

For example, if eNB₁ and eNB₂ belong to a node group recommended forsimultaneous transmissions, and eNB₁ transmits [+x −x] in two time slotsby applying an OCC of [+1 −1] to a reservation signal, eNB₁ may repeatthe transmission of [+x −x] in two time slots until before signaltransmission. Subsequently, if eNB₂ may sum signals in the two timeslots by applying an OCC of [+1 +1] orthogonal to the OCC of [+1 −1] andthen performs CCA, the reservation signal of eNB₁ is canceled in the twotime slots and thus reception power of the reservation signaltransmitted by eNB₁ may be excluded during CCA. If only eNB₁ transmits asignal around eNB₂, eNB₂ may perform signal transmission during achannel occupancy time, determining that the channel is unoccupiedduring a time period of transmitting the reservation signal. FIG. 10illustrates an example of performing CCA by applying an OCC.

A CCA threshold for OCC-based CCA may be configured independently of aCCA threshold for a general energy detection (ED) scheme.

As an additional operation of the present invention, it is proposed thata node of a node group capable of (or recommended for) simultaneoustransmission transmits a known signal by applying OCC₀ to the knownsignal, and when a CCA-performing node applies OCC₁ orthogonal to OCC₀during CCA, OCC₀ and OCC₁ are changed according to the time index of aspecific transmission unit. FIG. 11 illustrates a method for, when eNB₁and eNB₂ belong to a node group recommended for simultaneoustransmissions, changing an OCC pair (for example, OCC₀ and OCC₁) basedon an SF index according to a specific embodiment of the presentinvention.

If the additional operation is applied, the OCC-based CCA operation maybe designed in such a manner that even though an OCC is applied to aknown signal of a node of a different wireless business operator, theprobability of canceling the known signal is reduced, and thuscancellation of a known signal is possible only between nodes within anetwork of a single wireless business operator.

While the OCC-based CCA method has been described above as a method forapplying an OCC to N time slots, the OCC-based CCA method may beextended to an operation for transmitting a known signal by applying anOCC to N random orthogonal resources (e.g., frequency resources orspatial resources) and performing CCA by applying an OCC to the Northogonal resources.

Method for Sensing Collision with Network of Different Wireless BusinessOperator

According to a specific embodiment of the present invention, it isproposed that a sequence of a known signal is generated using anoperator ID as one of factors of a seed values. Herein, the known signalmay be a reservation signal (or preamble) which the node transmits forchannel occupation during a time period spanning from a time of CCAsuccess until before data transmission, or an RS (e.g., CRS, DM-RS,CSI-RS, or DRS) for channel estimation and data demodulation.

According to the afore-described OCC-based CCA method, in a situationwhere it is difficult to distinguish OCCs of different wireless businessoperators from each other due to an insufficient number of time slots, aCCA-performing node may cancel a reservation signal transmitted by anode within a network of a wireless business operator other than awireless business operator to which the CCA-performing node belongs,through OCC-based CCA. For example, if although eNB₁ and eNB₂ areincluded in networks of different wireless business operators, eNB₁applies the same OCC [+1 −1] for transmission of a reservation signal inFIG. 10, eNB₂ performs simultaneous signal transmission with eNB₁although eNB₁ is not for FR, as illustrated in FIG. 10.

Accordingly, as another method for distinguishing heterogeneous wirelessoperator networks in the embodiment of the present invention, it isproposed that a sequence of a known signal transmitted in an unlicensedband by an LTE node is generated using a wireless operator ID or anoperator ID as a seed value. Herein, a CCA-performing node may perform amodification of the afore-described OCC-based CCA method as follows.

Each of CCA-performing nodes performs at least one of the followingoperations.

-   -   The node sums signals in the N time slots by applying one OCC of        OCC group 2 to the N time slots and then performs power        detection (or energy detection) for CCA.    -   If power (or energy) detected by the above operation is lower        than a preset CCA threshold,        -   the node detects a known signal having an operator ID of an            operator to which the node does not belong.            -   If a known signal having an operator ID of an operator                to which the node does not belong is not detected, the                node transmits a signal during a channel occupancy time.            -   If a known signal having an operator ID of an operator                to which the node does not belong is detected,                -   the node attempts to detect a known signal of a node                    group to which the node belongs on a time slot                    basis.                -    Upon detection of the known signal, the node                    calculates reception power (or energy) from which                    reception power of the known signal is canceled in a                    corresponding time slot.                -    If the known signal is not detected, the node                    calculates reception power (or energy) for the                    corresponding time slot.            -   Subsequently,                -    if the sum of reception power (or energy) of each                    time slot for the N time slots is lower than a                    preset CCA threshold, the node transmits a signal                    after the last time slot.                -    If reception power (or energy) of the last time                    slot is lower than a preset CCA threshold, the node                    transmits a signal after the last time slot.    -   If the detected power (or energy) detected by the above        operation is higher than the preset CCA threshold,        -   the node may defer signal transmission (e.g., apply a            backoff), determining that the channel is occupied.

FIG. 12 illustrates an operation according to an embodiment of thepresent invention. In the method for channel sensing in an unlicensedband in a wireless communication system, the method of FIG. 12 may beperformed by a channel-sensing node.

The node may receive information about a known signal transmitted by atleast one other node of the same node group (S1210). The node may detectthe known signal using the information about the known signal (S1220).The node may cancel reception power of the known signal from measuredreception power, during a time period over which the known signal isdetected (S1230). The node may compare the remaining reception powerexcept for the canceled reception power of the known signal with athreshold (S1240). The node may determine whether a channel is occupiedor unoccupied according to the comparison (S1250). If the channel isoccupied, the node may transmit a signal during a channel occupancytime.

The information about the known signal may include at least one of asequence of the known signal, transmission power of the known signal, avalid duration of the reception power of the known signal, a poweroffset between a transmission signal and the known signal during thevalid duration, or a cancelation ratio of reception power of thetransmission signal during the valid duration.

Also, the valid duration may include a subframe in which the knownsignal is detected or a subframe carrying a signal of which thereception power may be derived from the reception power of the knownsignal.

Further, the known signal may include a reservation signal or preambletransmitted for channel occupation during a time period spanning fromchannel sensing success until before transmission of a signal during achannel occupancy time, or an RS for channel estimation and datademodulation.

If the information about the known signal includes information about thevalid duration or the known signal indicates the valid duration, thenode may cancel the reception power of the known signal from totalreception power during the valid duration.

If the known signal is transmitted in a plurality of OFDM symbols, theknown signal may be transmitted with predetermined phase differencesindicating the remaining lengths of a Tx burst to which the known signalbelongs in different OFDM symbols. In this case, the node may use thereception power of the known signal in calculating reception power forchannel sensing during a time period corresponding to the remaininglength of the Tx burst.

While the embodiments of the present invention have been described inbrief with reference to FIG. 12, an embodiment related to FIG. 12 mayinclude at least a part of the afore-described embodiment(s),alternatively or additionally.

FIG. 13 is a block diagram of a transmitting device 10 and a receivingdevice 20 configured to implement exemplary embodiments of the presentinvention. Referring to FIG. 13, the transmitting device 10 and thereceiving device 20 respectively include radio frequency (RF) units 13and 23 for transmitting and receiving radio signals carryinginformation, data, signals, and/or messages, memories 12 and 22 forstoring information related to communication in a wireless communicationsystem, and processors 11 and 21 connected operationally to the RF units13 and 23 and the memories 12 and 22 and configured to control thememories 12 and 22 and/or the RF units 13 and 23 so as to perform atleast one of the above-described embodiments of the present invention.

The memories 12 and 22 may store programs for processing and control ofthe processors 11 and 21 and may temporarily storing input/outputinformation. The memories 12 and 22 may be used as buffers. Theprocessors 11 and 21 control the overall operation of various modules inthe transmitting device 10 or the receiving device 20. The processors 11and 21 may perform various control functions to implement the presentinvention. The processors 11 and 21 may be controllers,microcontrollers, microprocessors, or microcomputers. The processors 11and 21 may be implemented by hardware, firmware, software, or acombination thereof. In a hardware configuration, Application SpecificIntegrated Circuits (ASICs), Digital Signal Processors (DSPs), DigitalSignal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), orField Programmable Gate Arrays (FPGAs) may be included in the processors11 and 21. If the present invention is implemented using firmware orsoftware, firmware or software may be configured to include modules,procedures, functions, etc. performing the functions or operations ofthe present invention. Firmware or software configured to perform thepresent invention may be included in the processors 11 and 21 or storedin the memories 12 and 22 so as to be driven by the processors 11 and21.

The processor 11 of the transmitting device 10 is scheduled from theprocessor 11 or a scheduler connected to the processor 11 and codes andmodulates signals and/or data to be transmitted to the outside. Thecoded and modulated signals and/or data are transmitted to the RF unit13. For example, the processor 11 converts a data stream to betransmitted into K layers through demultiplexing, channel coding,scrambling and modulation. The coded data stream is also referred to asa codeword and is equivalent to a transport block which is a data blockprovided by a MAC layer. One transport block (TB) is coded into onecodeword and each codeword is transmitted to the receiving device in theform of one or more layers. For frequency up-conversion, the RF unit 13may include an oscillator. The RF unit 13 may include Nt (where Nt is apositive integer) transmit antennas.

A signal processing process of the receiving device 20 is the reverse ofthe signal processing process of the transmitting device 10. Under thecontrol of the processor 21, the RF unit 23 of the receiving device 10receives RF signals transmitted by the transmitting device 10. The RFunit 23 may include Nr receive antennas and frequency down-converts eachsignal received through receive antennas into a baseband signal. The RFunit 23 may include an oscillator for frequency down-conversion. Theprocessor 21 decodes and demodulates the radio signals received throughthe receive antennas and restores data that the transmitting device 10wishes to transmit.

The RF units 13 and 23 include one or more antennas. An antenna performsa function of transmitting signals processed by the RF units 13 and 23to the exterior or receiving radio signals from the exterior to transferthe radio signals to the RF units 13 and 23. The antenna may also becalled an antenna port. Each antenna may correspond to one physicalantenna or may be configured by a combination of more than one physicalantenna element. A signal transmitted through each antenna cannot bedecomposed by the receiving device 20. A reference signal (RS)transmitted through an antenna defines the corresponding antenna viewedfrom the receiving device 20 and enables the receiving device 20 toperform channel estimation for the antenna, irrespective of whether achannel is a single RF channel from one physical antenna or a compositechannel from a plurality of physical antenna elements including theantenna. That is, an antenna is defined such that a channel transmittinga symbol on the antenna may be derived from the channel transmittinganother symbol on the same antenna. An RF unit supporting a MIMOfunction of transmitting and receiving data using a plurality ofantennas may be connected to two or more antennas.

The transmitting device and/or the receiving device may be configured asa combination of one or more embodiments of the present invention.

The embodiments of the present application has been illustrated based ona wireless communication system, specifically 3GPP LTE (-A), however,the embodiments of the present application can be applied to anywireless communication system in which interferences exist.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for performing channel sensing in anunlicensed band in a wireless communication system, performed by a nodeperforming channel sensing, the method comprising: receiving informationabout a known signal to be transmitted by at least one node of a groupto which the node performing the channel sensing belongs to; detectingthe known signal using the information about the known signal; cancelingreception power of the known signal from measured reception power,during a time period in which the known signal is detected; comparingremaining reception power except for the reception power of the knownsignal with a threshold; and determining whether a channel is occupiedor unoccupied according to a result of the comparison, wherein theinformation about the known signal includes at least one of a sequenceof the known signal, transmission power of the known signal, a validduration of the reception power of the known signal, a power offsetbetween a transmission signal and the known signal during the validduration, or a cancelation ratio of reception power of the transmissionsignal during the valid duration.
 2. The method according to claim 1,wherein the valid duration includes a subframe in which the known signalis detected or a subframe in which a signal of which reception power isto be derived from the reception power of the known signal istransmitted.
 3. The method according to claim 1, wherein the knownsignal includes a reservation signal or a preamble to be transmitted forchannel occupation after channel sensing is successful and beforetransmitting a signal during a channel occupancy time, or a referencesignal for channel estimation and demodulation.
 4. The method accordingto claim 1, further comprising, if the information about the knownsignal includes information about the valid duration or the known signalindicates the valid duration, canceling the reception power of the knownsignal from measured reception power during the valid duration.
 5. Themethod according to claim 1, wherein if the known signal is transmittedin a plurality of orthogonal frequency division multiplexing (OFDM)symbols, the known signal is transmitted with a predetermined phasedifference in different OFDM symbols to indicate remaining lengths of atransmission burst including the known signal.
 6. The method accordingto claim 5, further comprising using the reception power of the knownsignal in calculating reception power for the channel sensing during atime period corresponding to the remaining length of the transmissionburst.
 7. A node for performing channel sensing in an unlicensed band ina wireless communication system, the node comprising: a radio frequency(RF) unit; and a processor configured to control the RF unit, whereinthe processor is configured to receive information about a known signalto be transmitted by at least one node of a group which the nodeperforming the channel sensing belongs to, detect the known signal usingthe information about the known signal, cancel reception power of theknown signal from measured reception power, during a time period inwhich the known signal is detected, compare remaining reception powerexcept for the reception power of the known signal with a threshold, anddetermine whether a channel is occupied or unoccupied according to aresult of the comparison, and wherein the information about the knownsignal includes at least one of a sequence of the known signal,transmission power of the known signal, a valid duration of thereception power of the known signal, a power offset between atransmission signal and the known signal during the valid duration, or acancelation ratio of reception power of the transmission signal duringthe valid duration.
 8. The node according to claim 7, wherein the validduration includes a subframe in which the known signal is detected or asubframe in which a signal of reception power to be derived from thereception power of the known signal is transmitted.
 9. The nodeaccording to claim 7, wherein the known signal includes a reservationsignal or a preamble to be transmitted for channel occupation afterchannel sensing is successful and before a signal is transmitted duringa channel occupancy time, or a reference signal for channel estimationand demodulation.
 10. The node according to claim 7, wherein if theinformation about the known signal includes information about the validduration or the known signal indicates the valid duration, the processoris configured to cancel the reception power of the known signal frommeasured reception power during the valid duration.
 11. The nodeaccording to claim 7, wherein if the known signal is transmitted in aplurality of orthogonal frequency division multiplexing (OFDM) symbols,the known signal is transmitted with a predetermined phase difference indifferent OFDM symbols to indicate remaining lengths of a transmissionburst including the known signal.
 12. The node according to claim 11,wherein the processor is configured to use the reception power of theknown signal in calculating reception power for the channel sensingduring a time period corresponding to the remaining length of thetransmission burst.